In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. For example, the ink can be first transferred to an intermediate blanket that in turn is used to transfer the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful to prepare lithographic printing plates typically comprise one or more imageable layers applied over the hydrophilic surface of a substrate. The imageable layers include one or more radiation-sensitive components that can be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged regions or the non-imaged regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the imaged regions are removed, the element is considered as positive-working. Conversely, if the non-imaged regions are removed, the element is considered as negative-working. In each instance, the regions of the imageable layer (that is, the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Direct digital or thermal imaging has become increasingly important in the printing industry because of their stability to ambient light. The imageable elements for the preparation of lithographic printing plates have been designed to be sensitive to heat or infrared radiation and can be exposed using thermal heads of more usually, infrared laser diodes that image in response to signals from a digital copy of the image in a computer a platesetter. This “computer-to-plate” technology has generally replaced the former technology where masking films were used to image the elements.
These imaging techniques require the use of alkaline developers to remove exposed (positive-working) or non-exposed (negative-working) regions of the imaged layer(s). In some instances of positive-working lithographic printing plate precursors that are designed for IR imaging, compositions comprising infrared radiation-sensitive absorbing compounds (such as IR dyes) inhibits and other dissolution inhibitors make the coating insoluble in alkaline developers and soluble only in the IR-exposed regions.
Independently of the type of lithographic printing plate, lithography has generally been carried out using a metal substrate such as a substrate comprising aluminum or an aluminum alloy of various metallic compositions. The surface of the metal sheet is generally roughened by surface graining in order to ensure good adhesion to a layer, usually an imageable layer, that is disposed thereon and to improve water retention in non-imaged regions during printing. Such aluminum-supported imageable elements are sometimes known in the art as precursors to planographic printing plates or lithographic printing plates.
Various aluminum support materials and methods of preparing them are described in U.S. Pat. No. 5,076,899 (Sakaki et al.) and U.S. Pat. No. 5,518,589 (Matsura et al.).
In general, to prepare aluminum-containing substrates for lithographic printing plate precursors, a continuous web of raw aluminum is generally taken from unwind section through a degreasing section to remove oils and debris from the aluminum web, alkali etching section, a first rinsing section, a graining section that can include mechanical or electrochemical graining, or both, a second rinsing section, post-graining acidic or alkali-etching section, a third rinsing section, an anodization section using a suitable acid (such as sulfuric acid) to provide an anodic oxide coating, a fourth rinsing section, a “post-treatment” section, a final or fifth rinsing section, and a drying section, before either being rewound or passed on to coating stations for application of imageable layer formulations.
In the anodization section, the aluminum web is treated to form an aluminum oxide layer on its surface so it will exhibit a high degree of mechanical abrasion resistance necessary during the printing process. This aluminum oxide layer is already hydrophilic to some degree, which is significant for having a high affinity for water and for repelling printing ink. However, the oxide layer is so reactive that is can interact with components of the imageable layer in the imageable element. The aluminum oxide layer can partially or completely cover the aluminum substrate surface.
When sulfuric acid is used for providing the aluminum oxide layer, the resulting substrate can exhibit poorer adhesion to overlying radiation-sensitive compositions than if the substrate had been anodized using phosphoric acid. It is believed that the difference in adhesion can be caused by different anodic pore sizes created from the different acids used in anodization. That is, sulfuric acid anodization may produce smaller pores in the oxide layer.
Japanese Published Application 11-65096 (Fujifilm) describes a method for providing photosensitive lithographic printing plate precursors in which an anodic oxide layer is formed on the aluminum support, which anodic oxide pores have a diameter of 20 nm or less. A negative-working photosensitive layer is directly applied to this oxide layer.
U.S. Patent Application Publication 2002/0033108 (Akiyama et al.) describes the formation of oxide layers on aluminum supports to control the average size of the oxide pores in the range of from 6 nm to 40 nm. A water-receptive subbing layer can be applied over the oxide layer.
U.S. Pat. No. 7,078,153 (Hotta) describes a support having a predetermined vacancy ratio and micropores (vacancies) in its oxide surface.
After anodization, the substrate is typically “post-treated” with a suitable polymer to permanently seal the oxide pores so that components in the radiation-sensitive imageable layer do not enter the pores, and to further improve the hydrophilicity of the substrate surface so it better repels lithographic ink during the printing operation.
Commonly used post-treatment processes can include the reaction aluminum oxide with poly(vinyl phosphoric acid), or a mixture of sodium phosphate and sodium fluoride, to form a crosslinked hydrophilic layer on the substrate. To determine if pore sealing has occurred, the substrate can be dipped in an aqueous dye solution and rinsed. If little dye is seen on the substrate, the pores have been properly sealed.
It would be desirable to omit this post-treatment step of a sulfuric acid anodized aluminum-containing substrate that must be evaluated with an aqueous dye solution.
It is highly important to have good adhesion of the radiation-sensitive imageable layer to the underlying substrate. But it is also necessary to have rapid and complete removal of the exposed (positive-working) and non-exposed (negative-working) regions during development. These two important requirements often work against each other. It is very difficult to satisfy both requirements, especially when development is carrier out on-press where good adhesion of the remaining imaging layer is needed for it to survive thousands of printing impressions and complete removal of imageable layer should be accomplished within 50 printing impressions.
There is a need to improve both of these requirements especially when the sulfuric acid anodized substrates are used for the lithographic printing plate precursors that are on-press developable.